The present invention relates to coated cemented carbide metalcutting inserts, especially those of the indexable type.
The primary benefit of various coating materials to the metalcutting performance of coated indexable cobalt cemented tungsten carbide inserts has been well documented. Abrasion resistance (the most important consideration at lower cutting speeds) is provided by titanium carbide (or titanium carbonitride). Resistance to tool-workpiece chemical interaction (crater formation) is provided most commonly by alumina due to this material having a very low free energy of formation. Titanium nitride is reputed to lower tool-workpiece frictional forces and the occurrence of edge build-up. In addition, its lustrous gold color enhances the marketability of the coated tool and allows tool wear to be more readily observed.
The combined benefits of these materials have been used to advantage in first generation multilayer coated tool inserts. Examples include the coating systems titanium carbide/titanium carbonitride/titanium nitride and titanium carbide/alumina/titanium nitride. Additional benefits are expected for second generation multilayer coatings designed to achieve improved performance through optimization of layer thickness, number of layers and the sequence of layer deposition.
The advantages of reduced grain size in chemical vapor deposition (CVD) coatings applied to indexable cobalt cemented tungsten carbide inserts are well known. The most commonly utilized method of reducing grain size in alumina layers is to periodically interrupt the deposition of the alumina layer by depositing a thin layer of titanium carbide, titanium carbonitride or titanium nitride between the alumina layers. In this way, each succeeding alumina layer renucleates and grain growth is minimized. The advantages of this procedure were demonstrated by Dreyer and Kolaska ("Development and Tool Life Behavior of Super-Wear-Resistant Multilayer Coatings on Hardmetals," Metals Society (Book 278), London, England (1982), pages 112-117).
Considerable improvements in flank wear resistance have been observed when cutting hot-worked steel (54 HRC), chilled cast iron and Inconel 718 with commercially available multilayered alumina coated indexable inserts. These inserts utilized a ten layer coating consisting of titanium carbide, titanium carbonitride, titanium nitride and four layers of alumina separated by three layers of titanium nitride (Schintelmeister et al, "Cutting Tool Materials Coated by Chemical Vapor Deposition," Wear, 100 (1984), pages 153-169).
Improvements in both flank and crater wear performance have been demonstrated for a multilayer alumina coated insert with an initial 3 micrometer (.mu.) thick layer of titanium carbide overlaid by 19 layers of titanium nitride and 19 layers of alumina to a total thickness of 6 .mu.. When machine tested against conventional 6 .mu. thick titanium carbide coated and 5 .mu./1 .mu. thick titanium carbide/alumina coated inserts, the alumina multilayered insert demonstrated superior crater and flank wear resistance in the machining of C60 steel. Superior performance of this alumina multilayered coating was also observed during interrupted cutting of CK 45 KN steel (Dreyer et al, United Kingdom Patent Application No. GB 2048960A).
Applicants have now surprisingly discovered a multilayered coated cemented carbide cutting insert possessing an excellent combination of edge strength, deformation resistance, crater resistance and flank wear resistance, which make it commercially competitive in a wide range of metalcutting applications such as continuous and interrupted machining of carbon and alloy steels and gray and ductile cast irons.